In the fabrication of semiconductor devices on silicon wafers, various structures such as metalization layers, passivation layers, insulation layers, etc. are formed on a silicon substrate. The quality of the semiconductor device fabricated is a strong function of the processes with which these structures are formed. The quality is also a function of the cleanliness of the manufacturing environment in which the silicon wafer is processed.
Technological advances in recent years in the increasing miniaturization of semiconductor circuits require more stringent control of impurities and contaminants in the processing chamber of the semiconductor device. When the miniaturization of the device progressed to the submicron level, the minutest amount of contaminants can significantly reduce the yield of wafers.
Among the electronic materials frequently used for deposition on silicon wafers, silicon nitride has gained more importance in recent years. Silicon nitride is used extensively as a final protective passivation and coating layer for semiconductor wafers because of its excellent diffusion barrier characteristics against moisture and alkali ions. Silicon nitride is a desirable semiconductor material also for its high density and high dielectric properties.
The deposition of silicon nitride is usually performed by a low pressure chemical vapor deposition (hereinafter LPCVD) process in which dichlorosilane and ammonia are reacted together in a heated chamber. The reaction between silicon tetrachloride or dichlorosilane and ammonia normally takes place with nitrogen carrier gas at approximately 650.degree. C. or with hydrogen carrier gas at approximately 1,000.degree. C.
In a LPCVD process for the deposition of silicon nitride, the LPCVD chamber is first evacuated, a mixture of gases of silicon tetrachloride or dichlorosilane and ammonia is then introduced into the chamber which contains one or more silicon wafers each having a surface onto which a silicon nitride layer is to be deposited. The silicon wafers are generally heated to a deposition temperature at the time when the mixture of gases are fed into the chamber such that the gases decompose and thereby depositing a silicon nitride layer on the surface of the wafer. For instance, in a prior-art LPCVD system equipped with a horizontal boat that receives a plurality of silicon wafers positioned vertically in the chamber, reactant gases are injected into the chamber through a number of apertures and flow across the wafers. The use of such prior-art chambers encourages the growth of native oxide on the surfaces of the wafers.
Chemical native oxide is not a true silicon dioxide because it is not stoichiometrically formed. Native oxide layers are typically formed after a cleaning procedure partially due to the presence of moisture in the air. Native oxide is chemically different than grown silicon oxide which is intentionally deposited or formed. The physical properties of native oxide and silicon dioxide are also different, for instance, the refractive index of silicon dioxide is typically 1.45 while the refractive index for native oxide is approximately 2.2.
Native oxide also forms on wafer surfaces during prior processing steps which expose the wafer to ambient conditions. Some semiconductor processes include various cleaning steps prior to deposition of electronic materials. However, the cleaned wafers are usually still exposed to the ambient atmosphere after such cleaning and native oxide has the opportunity to grow on the wafer surface prior to the silicon nitride deposition.
For instance, the handling of multiple wafers in a wafer boat at multiple processing stations during a semiconductor fabrication process causes particular problems with regard to the formation of native oxide. The wafers take significant amount of time to load into the chamber, i.e. on the order of thirty minutes. During such loading step, air is present around the wafers and a native oxide layer readily forms on the newly cleaned surface of the wafer. This problem is compounded by the fact that such native oxide formation is not uniform, i.e., the first wafer in the chamber may grow a thicker layer of native oxide. This leads to a batch of integrated circuit structures having different electrical properties depending on the particular wafer from which the integrated circuit structures were formed.
It is therefore desirable that prior to the deposition of any layer of semiconductor materials on a silicon wafer, the surface of the wafer should be clean and free of contaminants such as native oxide or other impurities. Contaminants present at the interface between the silicon surface of the wafer and the layer formed thereon interfere with the electrical properties of the integrated circuit structures resulting in degraded performance or total failure of the structure.
It has been observed that the growth of silicon nitride films on a silicon wafer can be affected by the presence of native oxide on the silicon surface. This manifests itself as an "incubation time" during which growth of the nitride layer is retarded on the native oxide surface. A typical test for this incubation time can be conducted by depositing layers of silicon nitride under identical conditions but with varying deposition times. Longer deposition times correspond to thicker nitride films. A graph of the silicon nitride film thickness versus the deposition time therefore shows a straight line. This is shown in FIG. 1 as the solid line. The slope of this straight line represents the growth rate of the silicon nitride films.
Theoretically, the intercept at time equal to zero should show that the thickness of the nitride film is zero. However, in reality, thin films of silicon nitride exhibit a variable intersect of the horizontal axis at times from one to thirty or more seconds. The length of this time is referred to as the "incubation time", shown in FIG. 1 as A, since little growth of silicon nitride is observed during this time period. We have discovered that various process parameters affect the length of incubation time, as well as the thickness of native oxide on the silicon surface.
The incubation time has several detrimental effects in the formation of silicon nitride films on semiconductor devices. For instance, thin layers of silicon nitride are frequently used to form capacitor structures. The thickness control for thin silicon nitride dielectric films is therefore very surface and process dependent which in turn makes the capacitor control very difficult. Furthermore, a native oxide layer of variable thickness can reduce the capacitance of the dielectric layer, i.e. native oxide plus nitride, and further degrade the capacitor performance.
Others have attempted to clean a native oxide layer from the surface of a silicon wafer by a baking process at temperatures as high as 1,200.degree. C. However, such high temperature baking is only suitable in an epitaxial silicon growth process and not suited for a silicon nitride film process which is normally deposited at a much lower temperature.
It is therefore an object of the present invention to provide a novel method of in-situ cleaning native oxide from the surface of a silicon wafer that does not have the shortcomings of the conventional cleaning method.
It is another object of the present invention to provide a novel method of in-situ cleaning native oxide from the surface of a silicon wafer such that a clean interface is provided between a single crystal silicon wafer and a subsequently deposited semiconductor material.
It is a further object of the present invention to provide a novel method of in-situ cleaning native oxide from the surface of a silicon wafer such that the incubation period normally encountered in a silicon nitride film deposition process can be eliminated.
It is another further object of the present invention to provide a novel method of in-situ cleaning native oxide from the surface of a silicon wafer by heating the wafer to a modest temperature in an environment substantially free of oxidizing species such as oxygen and water.
It is yet another further object of the present invention to provide a novel method of in-situ cleaning native oxide from a silicon wafer surface by heating the wafer to a modest temperature in an environment substantially free of oxidizing species in the presence of at least one reducing gas.